One Punch Man and Mob Psycho 100 are both stories that originate as web-comics created by the author known as ONE. The author has enjoyed a period of success following the anime adaptation of One-Punch Man back in 2015, with the adaptation of Mob Psycho 100 dropping the following year.
While it is never a good thing to pit siblings against each other, and in this case, the siblings are as different in composition and outlook as Ritsu Kageyama is from his protagonist brother Shigeo. However, given the sheer acclaim offered to One-Punch Man, one would assume that it is the better story overall and achieves its objectives more than Mob Psycho 100 does, which is far from the truth. Here's the case for why Mob Psycho 100 is ONE's true magnum opus.
More Compelling Over-Powered Protagonist
Comparing the protagonists of One-Punch Man and Mob Psycho 100 is easy because they're just different manifestations of the same base concept - an overpowered main character. These characters are near omnipotent, but they do not revel in the extent of their power. One of them simply wants to live life for himself on his own terms while coming to grips with coming of age, while another wishes to engage in the kind of thrilling fight that would breathe some excitement into his life again. Both characters suffer inconveniences to some extent due to this omnipotence, but the scale and nature of this inconvenience is of course vastly different.
Saitama's abilities are all based on gags, so he is incredibly powerful due to something that's akin to toon physics. After running 10km, doing 100 push-ups and 100 sit-ups a day, Saitama lost his hair and gained insane power. Too much power giving rise to the titular issue – he defeats any enemy with a single punch. This has ruined the sensation of excitement, fear or panic which he'd experience in potentially dangerous situations prior to obtaining his incredible strength. Kageyama "Mob" Shigeo is an esper and one of the most powerful espers in anime. The extent of his abilities is truly unknown, but his true potential is unleashed when he is in a heightened emotional state, a state denoted by the appearance of a strange counter increasing throughout the course of an episode, culminating in an explosion of intense power and emotion by Mob when that counter reaches 100%.
Mob has had these abilities from his birth, so he has naturally had to grow up with powers the extent of which is not fully understood, which has had grave consequences for people around him, like his brother Ritsu. Saitama is one of anime's coolest characters and his resting face is one of ONE's funniest contributions, but the brilliance of their writing lies in how this particular face is reserved for his protagonists – two people in a league of their own in their respective universes. Such a position is incredibly unique, and their faces being so basic is to show us that these guys are really just anybodies in their respective universes, "mob characters" (background characters), while being hosts to unparalleled levels of power. These characters are still, like anybody else, just human beings, and they are caught up in the events stirred up by the dynamic and wonderful worlds built around them; however, Saitama's is less interested in changing himself and undergoing some kind of character arc than Mob is. Mob's goal in the series is clearly to take control of his own life and feel that he has the agency to do so. Mob works towards building himself in various respects from the first season, and this introspective nature of the character makes him one to be caught up in events more than initiate them.
He assumes the role of protagonist more proactively than Saitama does (or even has to). Mob, unlike the other espers in the show, wants to be a person defined by something other than those powers, which is why we see him struggle so hard to build his body, or to develop social skills to talk to the girl he likes, or go to a part-time job that doesn't pay very well to get esper advice from a scammer. The character still finds something to do even though they're in their special position in the greater scheme of things.
Better Side-Characters
From the Body Improvement Club, to Ritsu, Arataka Reigen, Dimple and several others, Mob Psycho 100 boasts a cast of more memorable side characters who assist Mob. The Body Improvement Club serves as Mob's first set of high school friends, his brother turns out to be like Mob's foil, possessing a level of competence in various things that Mob simply does not. Ritsu in turn has an inferiority complex regarding his brother's psychic powers and attempts to draw out his own psychic potential, which then brings the influence of the series antagonists into the mix. Mob's relationship with his brother is complex, as he too harbors an inferiority complex towards his brother for his social skills and academic excellence, which creates for far more compelling interactions and character motivations.
Concessions
For all its brilliant narrative, characters, and execution, Mob Psycho 100 is quite as hilarious as One-Punch Man, the out-and-out gag manga of the pair. With humour being the main goal of One-Punch Man, it achieves that more readily than Mob Psycho 100, a series that is funny in its own right, but humour is not always the objective. The fact that One-Punch Man is a gag anime/manga influences various elements of the story and therefore, while they pay various respects to shonen, both are perhaps better seen as two sides of the same coin, with one focused on humour and the other on drama and characterization. One-Punch Man with its superhero setting boasts a more interesting roster of villains than Mob Psycho 100 does, and this is a strength that Mob Psycho 100 could also have given the bizarre nature of its universe but it does not yet have the kind of villain that fans find memorable, at least not yet.